Low water pressure at one faucet: check the aerator first
When one faucet runs weak and the rest of the house is fine, the problem is at that fixture. Most of the time it is scale and grit in the aerator at the end of the spout.
How to tell what is causing it
If one faucet runs weak on both hot and cold while the others in the house are fine, the most likely cause is a clogged aerator. Delta's fix is to unscrew it, soak it in vinegar and scrub it with a toothbrush. See the aerator guide.
- Weak on both hot and cold, other faucets fine: aerator.
- Still weak with the aerator off: a stop valve under the sink not fully open, or a clogged cartridge.
- Weak on only hot, or only cold: that side's stop valve or the cartridge.
- Weak at every faucet in the house: not a fixture problem. Call your water utility or a plumber.
In the shower
For a Moen Posi-Temp shower with low flow at both the tub spout and the showerhead, Moen's fix is a new 1222 cartridge. If only the showerhead is weak, check the showerhead first.
Low flow from a new aerator may be by design: federal rules cap faucet aerators at 2.2 gpm, and WaterSense bathroom aerators run at 1.5 gpm or less.